Prior art measuring apparatus for measuring power in laser-beams having relatively high power, for example, about one Watt (1.0 W) has relied on use of a thermopile detector arrangement for converting energy from a laser-beam to be measured into heat, and estimating power from a rate of rise of temperature of the thermopile detector. Such a detector may include a receiving medium which absorbs light from the laser-beam and rises in temperature, a thermocouple array in thermal contact with the receiving medium for generating a voltage signal from the heating of the receiving medium, and suitable electronic arrangements for interpreting temperature rise of the receiving medium as a measure of laser power incident thereon.
Such measuring apparatus, in compact form, is prone to providing inaccurate or non-reproducible results for several reasons. By way of example, results may depend on the location of the measured laser-beam on the receiving medium, interpretation of which results is complicated in turn because temperature rise rate is typically non-linear. This is further complicated, as far as repeatability of measurements is concerned, by "memory" effects resulting from heating of the receiving medium in prior measurements. Where size and portability of such apparatus is not important, most of these problems can be overcome by heat dissipation arrangements including active cooling arrangements such as fans or the like. Arrangements such as this are still standard for laboratory measurements of laser power.
Size notwithstanding, however, thermopile devices are not readily adaptable to measuring laser-beams having a wide power range, for example six or more orders of magnitude. In order to effectively measure low powers the receiving medium must have a low thermal capacity, and consequently be mechanically somewhat fragile, in order to undergo a measurable temperature rise when irradiated by a low-power laser-beam. Such a fragile receiving medium is not acceptable for measuring a high-power beam. Attempts to attenuate a high power beam by a known factor before measurement only add to above-discussed inherent measurement inaccuracies.
Photon detectors, here, meaning devices which generate an electrical signal in response to light incident thereon, for example by direct generation of a voltage, as in a photovoltaic cell, or via a change in electrical resistance in response to light incident thereon, as in a photodiode, are inherently more accurate devices for laser power measurement. However, such devices have not been generally accepted as a means of measuring laser power over a relatively wide range. This is due in no small measure to that fact that such devices, photodiodes in particular, are easily saturated, and may be damaged, by even relatively modest levels of laser power. In this regard, such devices may be particularly vulnerable to laser-beams having a non-uniform cross-section intensity and which may include, in the cross-section, localized regions of high intensity, usually termed "hot spots" by practitioners of the laser art. This can lead to local damage as well as generally inaccurate measurements. The use of typical attenuating devices, such as thin-film, neutral-density filters, or absorbing glass attenuators, for reducing overall power in a laser-beam, by a known factor, prior to measurement, is at best partially helpful in this regard, as it does not deal with measurement inaccuracies resulting from the "hot spots", and the attenuators themselves are prone to damage by high-power beams. Photon detectors also have a response which is dependent on the wavelength of light being measured.
There is a need for a laser power measurement apparatus which can take advantage of the inherent accuracy of a photon detector device. The apparatus is preferably capable of measuring laser power over a relatively wide range of laser power and at a number of different laser wavelengths, and is preferably capable of being configured in an easily portable package.